Architecture of the myocardium
In the adult heart the myocardium of the atria is separate from that of the ventricles. There is, between the atria and ventricles, a fibrous partition, the upper and lower surfaces ol which give attachment to the muscle fibers of these cavities, respectively.
The fibrous partition is thickest in the triangle formed by the meeting of the aortic, and right and left atrioventricular ostia. This interval is filled by a mass of fibrous tissue, which in the angles between the aortic and the left atrioventricular ostium forms two thickened triangular masses, the trigona fibrosa. The fibrous mass is continued to the pulmonary ostium as the tendon of the conus. Below the point of junction of the trigona and the tendon of the conus these structures blend with the septum membranaceum ventriculorum. The septum membranaceum, tendon of the conus, and part of the trigona are derived from the aortic septum. The trigona give off laterally, on either side, atrioventricular rings which encircle the venous ostia and give attachment to the atrioventricular valves. There are also weak rings surrounding the pulmonary and aortic orifices; the aortic and left atrioventricular rings being partly confluent. The rings surrounding the arterial and venous ostia axe the annuli fibrosi.
The atrial musculature is attached to the trigona and atrioventricular rings only. The superficial fibers are attached to both rings and either encircle both atria in one loop, or enter the septum and form a figure 8. The deeper fibers are attached to one ring and encircle one atrium only; some fibers encircle only the auricle.
The ventricular musculature is very complex and consists of numerous superimposed layers distinguished from one another by the direction taken by the muscle fibers. In a general way, the fibers of the deepest layer have a direction crossing those upon the surface of the same area at a right angle. The intervening layers of fibers pass through all stages of obliquity.
1. AH fibers arise from, and are inserted into, the fibrous partition at the base. The attachment may be directly to the trigona or annuli, or indirectly to them by means of the chordae tendineae and atrioventricular valves.
2. The more superficial fibers tend to encircle the entire heart, passing over the longitudinal sulci. If they enter the septum they do so by passing into the vortex or whorl at the apex of the left ventricle. These fibers have always a definite direction upon the surface, i. e., from right to left upon the sterno-costal surface and from left to right on the diaphragmatic.
3. The deeper fibers all enter the septum in a direction oblique or perpendicular to its longitudinal axis. In addition, they completely encircle one or both ventricles forming, in the latter case, double loops.
During systole, as a result of this arrangement: (1) The papillary muscles and the longitudinal and antero-posterior axes of the ventricles are simultaneously shortened. (2) There is a movement of torsion or "wringing" which reduces the ventricular cavities to their minimum dimensions.